Rheostat



June 7, 1949. L. K. DAVIS 2,472,369

RHEOSTAT Filed Sept. 11, 1943 3 Sheets-Sheet 1 FIG./ ["7613 INVENTORLINCOLN K. DAV/5 ATTORNEY June 7, 1949. L. K. DAVIS 2,472,369

RHEOSTAT Filed Sept. 11, 1943 3 Sheets-Sheet 2 llilllmllllllllulmmullllllllllllllmllmlmu ii! a INVENTOR LINCOLN K. DA V/S ATTORNEY June 7,1949. V K. DAVIS 2,472,369

RHEOSTAT Filed Sept. 11, 1943 5 Sheets-Sheet 3 FIG. 14

80 a2 a2 84 FIG. 15

INVENTOR LINCOLN K. DA V/S ATTORNEY Patented June 7, 1949 UNITED STATESPATENT OFFICE RHEOSTAT Lincoln K. Davis, Brockton, Mann, assignor to theUnited States of America as represented by the Secretary of the NavyApplication September 11, 1943, Serial No. 502,070

12 Claims. 1

This invention relates to variable resistors or rheostats and moreparticularly to a novel device of this character in which variations inresistance are effected by a combination rolling and wiping action oftwo relatively movable contacting members. The new rheostat is of simpleconstruction, affords a good contact between the relatively movablemembers, and is rugged and resistant to wear.

Rheostats of the usual rotary or slide types are not entirelysatisfactory for the reason that the sliding movement of the contactover the resistance element causes considerable wear which may in timeresult in poor contact or breakage.

One object of the present invention, therefore, resides in the provisionof a novel rheostat which is relatively free from wear by reason of itsinclusion of a contact member adapted to roll over the resistanceelement with a wiping action to vary the effective resistance. I havefound that by the use of a rolling contact which affords a slight wipinaction, the friction incident to adjustment of the rheostat may beconsiderably reduced without sacrificing the desired firmness ofcontact.

Another object of the invention is to provide a rheostat of novelconstruction which may be manufactured at low cost. is sensitive toadjustment and afiords a good electrical contact at all adjustments.

A further object of the invention is to provide a rheostat having meansfor pivotally mounting the resistance element and the contact member sothat the region of contact remains out of alignment with the centers ofmovement of the two parts during pivotal movement thereof, whereby suchmovement results in a combination rolling and wiping action of thecontact member on the resistance element.

Still another object of the invention resides in the provision of arheostat wherein the contact member is in the form of a roller movablealong the resistance element at a speed which varies from the linearspeed of part of the roller surface contacting the resistance element,so that the rolling movement is accompanied by a wiping action of theroller relative to the resistance.

These and other objects of the invention may be better understood byreference to the accompanying drawings. in which Fig. 1 is a side viewof one form of the new rheostat;

Fig. 2 is a sectional view on the line 2-2 in automatic control deviceassociated therewith;

Fig. 'I is a similar view of part of a modified form of the controldevice;

Fig. 8 is a side view of part of another form of the rheostat;

Fi 9 is a sectional view on the line 8-9 in Fi 8;

Fi s. 10, 11 and 12 are side views of modified forms of the rheostatshown in Fig. 8;

Fig. 13 is a side view of the rheostat applied to an electric motor tocontrol the motor in accordance with the torque, and Y Figs. 14 and 15are similar views showing different forms of the rheostat applied to amotor to control the speed.

Referring to the drawings, the rheostat shown in Fig. 1 comprises aresistance coil 20 wound on an insulating bar 2|. At its upper-end, thebar has a laterally extending arm 2: pivotally connected to a stud 22secured to a suitable support 23. Adjacent the resistance coil is anelongated contact member having an arcuate face 24a normally engagingone face of the coil, the lower end of the contact member 24 beingpivotally connected to a support 25, as shown at 26. The contact face24a is urged against the adjacent face of resistance coil 20 by atension spring 21 connected between the contact member 24 and the freeend portion of bar 2|.

The contact member 24 is movable on pivot 20 by means of an operatingshaft 28 pivotally connected to the upper end of the contact member.When the member 24 is swung about pivot 26. its curved face 24a rollsalong the adjacent face of resistance coil 20, and due to the spacedrelation of the pivots for the contact member 24 and the resistance bar2|, respectively, the rolling is accompanied by a sliding action of thecontact face 24a over thecoil. As a result, a good contact is maintainedbetween the curved face 24a and the coil, and the sliding action tendsto remove any foreign matter'which might otherwise accumulate betweenthe contacting faces and thereby impair the conduction of current. Itwill be apparent that as the contact face 24a rolls along the resistancecoil, the effective value of the resistance increases or decreases,depending upon the direction of movement of contact member 24 and the.manner of connecting the resistance coil in circuit. The spring 21maintains a firm engagement of the contacting surfaces at all times.

A significant feature of the new rheostat is that the region of contactbetween the curved face 24a and the resistance coil is always maintainedin offset relation to a line between the pivots or centers of movementof the contact member and the resistance, respectively, throughout therange of adjustment of the rheostat. Because of this positioning of thepivot points with respect to the contacting faces,-rolling of the face24a along the resistance coil incident to adjustment of the rheostat-isalways accompanied by a slight wiping or sliding action. It will beapparent that with the construction shown in Figs. 1 and 2 only a slightmovement of the shaft 28 is necessary to operate the rheostat throughits full range, since each of the levers 2| and 24 is pivotally mountedat the end remote from the pivoted end of the other lever.

The rheostat shown in Fig. 3 comprises a frame 38 supporting aresistance coil 3| wound on a curved insulating bar 32. At itsends, thebar 32 is connected to the frame by brackets 33 held by screws 34. Theresistance coil 3| coacts with a contactor 35 in the form of a thinmetal ribbon which is normally curved transversely so that it istrough-shaped. At itsupper' end, the ribbon 35 is connected to thebracket 33 by means of a rivet 36 extending through the bracket, theinsulating bar 32 and the ribbon, the later being suitably insulatedfrom the rivet and the bracket, as shown at 36a. The lower end of theribbon 35 is connected to one end of an operating lever 31 movable on apivot 38. A tension spring 39 is connected between the outer end oflever 31 and an arm 48 on the frame, so that the ribbon 35 is normallyheld in tension against the curved face of resistance 3|.

Under the tension of spring 39, the ribbon 35 is wrapped along theresistance coil 3|, and the resulting imposition of a lengthwisecurvature in the ribbon tends to reduce its transverse curvature andcause the ribbon to lie flat against the resistance. When the lever 31is operated to draw the ribbon more tightly against the resistance, theresulting transverse flattening of the part of the ribbon moving intoengagement with the resistance causes the edges of the ribbon to slideover or wipe the resistance transversely, whereby the desiredcombination of wiping and rolling actions is obtained. It will beunderstood that when the outer end of lever 31 is moved downwardly, theeffective value of the resistance 3| is changed due to movement of theribbon progressively out of contact with the intermediate portion of theresistance. Conversely, when the outer end of lever 31 is movedupwardly, the effective value of resistance 3| undergoes an oppositechange for the reason that an additional part of the ribbon is drawnagainst the adjacent face of the resistance.

Referring to Fig. 6, I have'shown a rheostat made in accordance with myinvention and arranged to control a circuit including a variable 4 load.The rheostat, as shown, is generally similar to that illustrated in Fig.1 and comprises a resistance coil 40 wound on an insulating bar 4|pivoted at one end on arm 42. The coil 48 cooperates with a contactingmember 43 having a curved face 43a normally engaging one face of thecoil. The contacting member 43 is pivotally mounted on an arm 44 nearthe free end of resistance bar 4|, and the member 43 projects beyond thearm 44 where it is connected to a tension spring 45 secured to an arm45. A compression spring 41 is interposed between the resistance bar 4|and a frame 48a and normally urges the resistance coil 48 against thecurved face of contact 43. Y

At its free end, the contact member 43 carries an armature 48 adjacent apole piece 49 of an electromagnet 58. The electromagnet is supported ona bracket 5| secured to the frame 46a and has an adjustable stop 52 forlimiting movement of the arm 43 away from pole 49. The armature 48 isconnected to the positive side of a current source 53, the negative sideof which is connected through the load, which may be a motor 54, to oneside of the winding of electromagnet 58. The other side of the windingis connected through a wire to the outer end of resistance coil 40.

In operation, assuming that there is an increase in the current load dueto operation of motor 54, the electromagnet 50 draws armature 48 towardthe pole 49. As a result, the curved contact face 43 rolls to the rightalong resistance coil 40 with a wiping action, as previously described,so that the effective resistance of coil 48 is increased to offset theincrease in current through the circuit. Conversely, when the current inthe circuit decreases due to operation of the motor 54, armature 48moves away from pole 49 under the action of springs 45 and 41 and causesthe curved contact face 43a to roll to the left along resistance 48,whereby the effective resistance of the coil 48 is decreased to offsetthe change in current.

The device shown in Fig. '7 is similar to that illustrated in Fig. 6,except that the pole of the electromagnet and the armature on thecontact member 43 are of different form. As shown in Fig. '7, theelectromagnet 50a has a pole 49a pro vided with a bevelled face lyingadjacent to a parallel bevelled face of an armature 48a on the free endof contact member 43. By thus shaping the pole and the armature, thedisplacement of the latter may be caused to have the desired relation tothe change of resistance. It will be apparent that the opposing faces ofthe armature and pole may have any other desired form, depending uponthe desired relation of armature displacement to resistance variations.

In Figs. 8 and 9, I have shown another form of the rheostat whichcomprises an arcuate resistance element51 having a curved face normallyengaged by a contact roller 58. The roller is mounted on a shaft 59journalled on an arm 68 I having a bifurcated end portion 68a forreceiving the ends of the shaft. The arm 60 is mounted on an operatingshaft 8| rotatably mounted in a frame 62. A gear 83 rotatable on theshaft 59 is integral with the roller 58 and meshes with a rack 84 on theframe, the radius of the gear 53 being somewhat greater than the radiusof the roller 58.'

In operation, when arm 68 is rotated on shaft 6|, the gear 83 is rotatedby its engagement with rack 64 and drives the roller 58. Due to thedifference in the radii of the gear and the roller, the face of theroller contacting resistance 51 moves along the resistance at a speedsomewhat less than the linear velocity of shaft 59, and, therefore, theroller moves along the resistance with the desired rolling and wipingaction. By changing the ratio of the radii of the gear and roller, thedegree of wiping action may be varied as desired.

A modified form of the rheostat is illustrated in Fig. 10. As thereshown, the rheostat comprises a generally circular resistance strip 88mounted on a stator 81 made of insulating material. The resistance 68 iswound on an insulating bar 860 secured at its ends to the stator byscrews 88. A shaft 68 is rotatable in the stator and carries a radialarm Iii, and a roller ii is mounted on apin 12 on the free end of thearm. The roller projects partly beyond the resistance strip 88 andhasone flat side face engaging the strip. Accordingly, when the arm Iii isrotated on shaft 88, the roller II rolls along the resistance strip witha sliding or wiping action thereon.

The rheostats shown in Figs. 11 and 12 are somewhat similar to thatillustrated in Fig. 10. However, in the rheostat shown in Fig. 11, thesliding or wiping action is obtained by mounting the roller Ila on athreaded arm Ilia, the roller having internal threads engaging theexternal threads on the arm. Thus, when the arm a is rotated on shaft88, the roller moves along resistance strip 88 and also moves radiallyrelative to the strip, the radial movement providing the desired slidingor wiping action. The range of radial movement of the roller along thearm 10a is lim-.

ited so that the roller always makes contact with the resistance stripwithin the range of adjustment of the rheostat. In the rheostat shown inFig. 12, the radial arm 10b carries at its free end a roller 1 lb, theaxis of which is inclined relative to the arm. By reason of the skewmounting of the roller on the arm, movement of the latter results in acombination rolling and wiping action of the roller on resistance 88.

Referring to Fig. 13, I have shown one form of the new rheostat arrangedto control a motor 14. The rheostat comprises a stator 15 mounted on androtatable with the motor shaft and supporting a generally circularresistance element 180. A load shaft 16 is mounted in alignment with thedrive shaft of the motor but is not connected directly therewith, theconnection being made through a coil spring 11. r The rheostat includesa roller arm 18 projecting radially from the load shaft 18 and carryinga skew-mounted roller 18 similar to the roller II b, the periphery ofthe roller engaging one face of resistance 15a. The resistance 15a isconnected in circuit with the armature winding of the motor by suitablewiring (not shown) extending through the motor shaft.

In operation, an increase in the load on shaft 18 causes roller 19 tomove in one direction along resistance 15a with a combination rollingand wiping action, with the result that the effective resistance in thearmature circuit is changed to compensate for the increased load.Conversely, when the load on shaft 18 is decreased, the roller 18 movesin the opposite direction along resistance 18a so that a compensatingchange is effected in the resistance of the armature circuit. It will beapparent that the drive spring 11 permits relative rotational movementbetween the motor shaft and the load shaft in accordance with variationsin the load.

A modified form of the motor control is illustrated in Fig. 14. As thereshown, the drive shaft of the motor 14 carries a head 88 rotatable withthe shaft. A resistance strip 8i is pivotally mounted at one end on thehead 80 at one side of its axis, and a contact "member 82 is pivot'allymounted on the head at the oposite side of its axis. The contact member82 has a curved face 82a which is normally held against one face of theresistance strip 8| by a tension spring 88 connected at its ends to thecontact member and the resistance strip, respectively. A weight 84 ismounted on the free end portion of contact member 82 on one side of theaxis of rotation of the head 80. In operation, an increase in the motorspeed causes the free end of contact member 82 to move outwardly fromthe axis of rotation under the action of centrifugal force on the weight84. As a result, the resistance strip 8i moves with the contact memberbecause of its connection thereto through the tension spring 83, theresistance strip moving about its pivot on the head 88. The resultingrelative movement of the resistance strip 8| and the contact member 82causes the curved face 82a of the contact member to roll alongresistance 8| with a wiping action. The resistance strip 8| and contactmember 82 may be connected by suitable wiring (not shown) into thearmature circuit of the motor so that changes in the effectiveresistance of the strip 8i, due to variations in the speed of the motorand the centrifugal force on weight 84, effect a compensating action inthe armature circuit of the motor.

I have shown in Fig. 15 a governor 88 mounted on the drive shaft of theelectric motor 14 and operatively connected to the free end of a forkedlever 81. The lever 81 is pivotally mounted on a pin 88 in a frame 88,which is made of insulating material, and has a curved face 810.normally engaging one face of a resistance strip 98. One end of theresistance strip 80 is supported on a pivot Si in the frame, and thefree end of the strip is engaged by one end of a compression spring 92seated on a conducting member 92a on the frame, whereby the resistancestrip is urged against the curved face of lever 81. The lever 81 isconnected through a wire 93 to one side of a current source 94, theother side of the current source being connected to one side of thefield winding (not shown) of motor 14. The other side of the fieldwinding is connected through a wire 85, the conducting member 92a andspring 82 to the lower end of the resistance strip,

When the speed of the motor 14 increases, the weights on governor 86cause lever 81 to move to the left on pivot 88, this movement of thelever causing resistance strip 90 to moveto the left on its pivot 8|against the action of spring 92. Accordingly, the curved face 81a of thelever rolls upwardly along resistance strip 90 with a sliding or wipingaction so that the effective resistance of the strip is increased. Thus,the current through the field circuit, including resistance 98 andspring 92, is decreased to compensate for the increase in the motorspeed. When the speed of the motor decreases, the lever 81 is moved inthe opposite direction and causes the curved contact face 81a. to rolldownwardly along resistance 98 with a wiping action so as to decreasethe resistance in the field circuit and compensate for the decrease inmotor speed.

In the control devices shown in Figs. 14 and 15, suitable means (notshown) are preferably provided so that the rheostat does not begin tooperate until the motor has nearly attained the deto cut in only whenthe motor speed reaches a predetermined maximum.

1 claim:

" 1. In a rheostat, a resistance member, a contact member engaging saidfirst member, the contact member and resistance member being movablerelatively to' vary the effective resistance of said first member, andconstraining means for moving said members relatively to cause one ofthe members to move along the other with a com- .bination rolling andsliding action, the constrainin means including pivots about which said.members are movable and disposed so that a line between the pivots isoffset from the point of contact of the members throughout the range ofmovement thereof. I, 11' 2. In a rheostat, a curvedresistance strip, athin metal ribbon adjacent the strip, the ribbon being generallytrough-shaped with its concave ,side facing said strip, and means forwrapping .the ribbon progressively along the strip, whereby the ribbonstraightens transversely in the region ,of contact with the strip tocause the edges of the ribbon to wipe the strip, I I v 3. In a rheostat,a resistance strip, a contact arm having a free end portion movablealong said strip, a roller turnably mounted on the contact arm andperipherally engaging the resistance strip, and means operativelyconnected with the roller whereby the peripheral face of the rollercontacting the strip moves along the strip at less Ethan the linearvelocity of its axis thus to perform a wiping action as it is rolledalong the strip by movement of the arm.

' 4. In a rheostat, a generally circular resistance strip, a contact armadjacent the strip and pivotally mounted on the axis of curvaturethereof, and a roller mounted on the free end portion of the :arm androtatable thereon about an axis parallel to the axis of curvature of theresistance strip, 'one fiat side surface of the roller engaging said.strip.

1 5. In a rheostat, a generally circular resistance strip, a contact armadjacent the strip and pivot- 'aliy mounted on the axis of curvaturethereof, and

.a roller threaded on the free end portion of the arm with theperipheral portion of the roller engaging the strip and with the axis ofthe roller disposed generally transversely of the strip, whereby theroller is screwed generally transiversely of thestrip as it rollsalongthe strip on the 6. In a rheostat, a generally circular resistancestrip, a radial-arm pivoted at the axis of curvature of the strip, aroller engaging the strip, and means for rotatably mounting the rolleron the free end portion of the arm with the axis of the roller disposedat an acute angle to the longitudinal axis of the arm, whereby theroller is .movable on the arm along the strip with a combination rollinand sliding action.

7. In a rheostat, a curved resistance strip, a curved rack adjacent thestrip, an arm pivotally mounted on the axis of curvature of the strip, aroller mounted for rotation on the free end portion of the arm with theperiphery of the roller engaging said strip, a gear mounted for rotationon the free end portion of the arm and meshing with the rack, the radiiof the gear and the roller bein of different lengths, and an operativeconnection between the gear and the roller whereby movement of the armcauses the roller to rollalong said strip with a sliding action due tooperation of the gear.

8. In a circuit having'a variable load, an electromagnet, a resistanceelement pivotall mounted at one end and a current source connected inseries with the load, a contact arm pivotally mounted at one end portionand having a curved face engaging the resistance element, said pivotsbeing disposed so that a line between the pivots is offset from thepoint of contact of the members throughout the range of movement thereofand the arm being movable to cause said face to roll along theresistance element with a sliding action to vary the effectiveresistance thereof, and an armature on the free end portion of the armand disposed adjacent one pole of the electroma net for moving the armin accordance with variations in the current through the electromagne'tdue to changes in the load.

9. In a circuit having a variable load, an electromagnet, a resistanceelement and a current source connected in series with the load, acontact arm pivotally mounted at one end portion and having a curvedface engaging the resistance element, the arm being movable to causesaid face to roll along the resistance element with a sliding action tovary the effective resistance thereof, and an armature on the free endportion of the arm and disposed adjacent one pole of the electromagnetfor moving the arm in accordance with variations in the current throughthe electromagnet due to changes in the load, said pole and armaturehaving their opposing faces bevelled. 10. In combination with anelectric motor having a drive shaft and a load shaft operable by thedrive shaft, a rheostat assembly for controlling the motor in accordancewith variations in the load and comprising a resilient connectionthrough which the load shaft is driven by the drive shaft, whereby theshafts are rotatable relatively upon variations in the load on said loadshaft a generally circular resistance element mounted on one of theshafts and rotatable therewith, a radial contact arm mounted on theother shaft and rotatable therewith, and a roller on the free endportion of the arm enga ing the resistance element and movable thereonwith a combination rolling and sliding action in re.-

sponse to relative rotational movement of the mounted at one end on thehead on the opposite side of said axis, the arm having a curved faceengaging the resistance, a spring normally urging the resistance elementand the arm together, and a weight on the arm operable by centrifugalforce to move the free end portion of the arm 65,

resistance element, thereby causing said curved face to roll along theresistance element with a sliding action and vary the effectiveresistance outwardly against the centrifugal force on the of saidelement.

12. In combination with an electric motor havshaft, a pivoted armoperatively connected to the governor and having a curved face, aresistance 9 10 element pivoted adjacent the arm and normally M8 118 thecurved face thereof, and means for REFERENCES CITED connecting saidelement in circuit with the motor The following references are or recordin the including a wire leading from said arm and a me Of this P nspring normally urging the resistance element 5 against the arm, saidarm being movable by the UNITED STATES PATENTS governor to cause thecurved face to roll along Number Name Date the resistance element with asliding action and 8 9.7 8 Bilur Dec. 25. 1906 thereby vary theeffective resistance oi said ele- 8, 4 Cook July 1, 1919 ment. l0 1,64,68 Bentley Jan. 4, 1921 2,315,277 Shaw Mar. 30, 1943 LINCOLN K.DAVIS. 33,47 DQ515011 "Nov. 2, 1943

